Method for base sequencing and biologically active nucleic acids

Aptamers are nucleic acids and similar molecules, such as peptide-nucleic acids, that specifically bind to a ligand such as a protein or peptide. The present invention provides aptamers comprising at least one base capable of base pairing and different from the standard Watson-Crick bases. The present invention also relates to a method for preparation of such aptamers and to methods for sequencing nucleic acids that comprise at least one base capable of base pairing and different from the standard Watson-Crick bases.